Fibers using a thermoplastic polymer such as polyester or polyamide have excellent mechanical properties and size stability. Accordingly, such fibers are industrially valuable, and they are widely used not only for garment use, but also for interior use, automobile interior use, and other industrial uses.
However, demands for the textile material is diverse where comfort and convenience are simultaneously pursued, and single fibers comprising a conventional polymer could not always cope with such demands. Designing a new polymer from scratch to cope with such demands is too costly and time-consuming, and use of a conjugated fiber having the features of two or more polymers has been a frequent choice. A conjugated fiber can be provided with properties that cannot be realized in a single fiber, for example, by covering the main component with another component. Accordingly, various conjugated fibers having various shapes have been proposed, and various techniques have been proposed depending on the intended use of the fiber.
Of the conjugated fibers, core-sheath conjugated fibers produced by covering a core component with a sheath component are often used in applications where effects such as texture and bulkiness and mechanical properties such as strength, modulus, and abrasion resistance that could not be realized by a single fibers are pursued. Use of such core-sheath conjugated fiber enables production of a fibers having special cross-sectional shape that could not be realized by a nozzle for single fibers. When the melt spinning of a polymer such as a polyester or polyamide is conducted, the polymer discharged from the spinning nozzle experiences high surface tension in the course of its cooling, and since the cross-section becomes circular because of the higher stability, production of a fiber having a highly complicated cross section is difficult. In the meanwhile, development of a core-sheath conjugated fiber is one direction of fiber development since a fiber having a special cross-section enables production of a fiber having unique texture that could not be realized by the fiber having a circular cross-section with increased contact area with the additional resin coating the fiber, namely, production of a fiber having various functions from the same polymer.
Examples of the fiber having a special cross-section include those proposed in Japanese Unexamined Patent Publication Nos. 2004-339616 and 2008-7902 prepared by applying the core-sheath conjugated fiber, namely, by using the technique of forming a fiber having slit-shaped continuous grooves formed in the direction of the fiber axis.
JP '616 proposes a deodorant fiber wherein slits are formed in the surface layer to increase contact area with the air compared to conventional fibers having a normal circular cross-section and, in this fiber, improved deodorant function is realized by forming such fiber from a polymer such as phosphate salt having a deodorant function.
In addition to the use of the thermoplastic polymer having a deodorant function, JP '616 attempts to realize the merit of enhancing the deodorant function by providing 20 or more slits each having a depth at least twice the groove width in the surface layer of the fiber to thereby increase surface area per unit weight of the fiber (specific surface area).
However, many deep slits reaching the inner layer of the fiber are formed in the fiber of JP '616 since increase in the specific surface area is the primary aim of JP '616 and, accordingly, that fiber may enjoy excellent initial performance when the slits are still retained. However, when the fiber is used for garment textile application wherein the fiber is subjected to abrasion and repetitive complicated deformation, provision of many deep slits becomes problematic. In other words, in JP '616, the slits are in the form of deep grooves, and formation of the projections having a shape highly durable to the abrasion is not considered and, therefore, projections formed on the surface layer of the fiber are peeled off from their base by abrasion and the like, and the peeled projections in the form of fine fluffs may adversely affect the feel and the color development and, also, the deodorant function realized by the slits may be degraded in a large way with lapse of time.
JP '902 proposes a fiber having many fine slits formed in its surface layer pursuing sharp multi-shaving effects and inner wrapping effects to provide excellent wiping performance and polishing performance with the fiber.
JP 902 uses a fiber having a diameter apparently the same as that of the conventional fibers, but formed with many fine slits, and the wiping cloths produced by using such fiber has the possibility of exhibiting the performance equivalent to the wiping cloths produced by using conventional ultrafine fibers without sacrificing the mechanical properties such as fiber strength.
However, as in JP '616, the slits of JP '902 are wedge-shaped and extend also extremely deeply to the inner layer of the fiber. Accordingly, the slits are easily peeled off when the fibers are repetitively abraded, and the fabric made from such fiber are likely to experience loss of the wiping performance in repeated use by generation and falling of the fluffs by the peeling of the projections, while there may be some possibility of using the fabric for a disposable wiping cloth. Use of such fiber for the garment textile which is likely to experience scratching and repetitive deformation in the practical use is extremely difficult.
The technology proposed in JP '616 and JP '902 pursued increase in the specific surface area of the fiber, and their use in the application where the fabric experiences scratching, abrasion, and repetitive deformation as expected for general application of garments and industrial materials was difficult although there may be possibility of using such technology in limited applications under limited conditions. More specifically, use of such technology in garment textiles where texture, feel, and color development are critical is particularly unsuitable.
Fibers for garment textile application having the slit morphology are disclosed in Japanese Unexamined Patent Publication Nos. 2004-52161 and 2004-308021. The selling points of these fibers are textures and color development realized by slit morphology.
JP '161 and JP '021 propose a technology of providing many slits having a depth of at least 2 μm in the surface layer of the fiber as a fiber capable of exhibiting squeaky texture like that of natural silk fiber and expressing a deeper color tone.
In JP '161 and JP '021, slit mobility during the crumpling and deformation in the compression direction is realized by provision of the slits each having a depth of at least twice the groove width, and the squeaky texture is realized by the thus increased friction between the fibers. It is also disclosed that the fine slits in the surface layer of the fiber suppress light diffusion on the surface layer of the fiber to realize development of deeper color tone.
While use of the fibers in the garment textile application is intended in JP 161 and JP '021, the technology disclosed therein cannot be regarded as a technology that have considered the subsequent process with repetitive application of a relatively high stress or a technology where slits with the morphology durable to the abrasion or repeated use are provided. More specifically, in the core-sheath conjugated fiber having a special cross-section, the sheath component would be peeled off by friction with the yarn guide or the reed, or the slits would be broken during the dissolution of the sheath component since the fabric undergoes complicated deformation in the treatment bath, and this may adversely affect the texture and the color development. In addition, the projections of the slits weakened by fatigue in the subsequent process is easily peeled off in the actual use to produce fine pillings, and the abraded part exhibits poor texture with rough feeling, and this invites marked loss in the quality of the fabric. Furthermore, the deeper color tone pursued in JP 161 and JP '021 is greatly harmed by diffusion of light caused by fluffs and partial whitening. As described above, most conventional slit fibers failed to consider durability in the subsequent processing and in the practical use, and they were associated with problems in actual use. Accordingly, there have been demands for a fiber having special cross-section provided with two or more slits in the surface layer, and a core-sheath conjugated fiber for producing such fiber having special cross-section at a high productivity exhibiting excellent processability in the subsequent process.